Set column names to field.names in getSymbols.MySQL/SQLite

The documentation says that `field.names` are the "names to assign to returned columns", but that does not actually happen. It's simple enough to use the `field.names` (if non-NULL) when setting the column names. Thanks to Cedrick Johnson for the suggestion/patch.

Also consider renaming `field.names` to `col.names` to be consistent with `getSymbols.csv/rda/rds`.

Locate the getSymbols.MySQL and getSymbols.SQLite implementations and read the documentation describing field.names. Verify how returned column names are currently assigned, then make field.names take effect when non-NULL and confirm the behavior for both database backends; consider the proposed col.names rename separately because the issue presents it as an additional consideration.

Written by the indexing model from the issue text.